The NI Crossword

Curiosities
Questions that have always intrigued you about the world will appear in this, your section,
and be answered by other readers. Please address your answers and questions to ‘Curiosities’.

Just how dangerous is DDT? If it is as dangerous as some people suggest, why is it still being used in countries like Namibia? What is behind this strategy?

DDT passes through all links of the food chain, beginning as a dusting over crops and ending in milk and butter. Residues of DDT are little affected by washing. It is an organochlorine compound which, although discredited, remains in our environment as it accumulates in the body fat of fish, animals and ultimately humans, causing cancers and other health problems. It is now banned in many countries but still makes profit for the chemicals industry peddling it in the Third World. Of interest may be an information sheet entitled WHO Murdered Africa from Nemesis, 1 Quarry Bank Road, Chesterfield S41 OHH, England.

Sheila Edwards
Dubai, United Arab Emirates

There is no straightforward answer. DDT has a very bad public image, which may be both deserved – and exaggerated. It is a persistent insecticide, which hangs around in the environment for a long time and has the ability to build up in food chains. Just how persistent depends on temperature, sunlight intensity or rainfall.

In terms of wildlife, it can be classified as dangerous. It’s toxic to fish and a range of invertebrates, can interfere with oestrogen production in fish and may have effects on hormonal balance and fertility in other animals as well.

Its use in agriculture in the US and Europe led to population decline in birds of prey – but populations recovered a decade after use stopped. It also led to imbalances in the populations of insects, causing severe pest outbreaks.

Even used very carefully in Zimbabwe to control tsetse flies, it harmed woodland birds, lizards and birds of prey. (See RJ Douthwaite & CCD Tingle, DDT in the Tropics: The impact on wildlife in Zimbabwe of ground-spraying for tsetse fly control, 1994, Natural Resources Institute ISBN 0 85954 364 1. Available from CAB International, Wallingford, Oxon OX10 8DE, UK)

However, toxicity to humans is actually rather low. A single, oral dose of 10 mg/kg is sufficient to cause illness in some but not all people. The WHO classes it as ‘moderately hazardous’. Young babies whose mother’s milk contains high levels of DDT have slower reactions. Interference with thyroid activity has also been shown, with resulting disturbance of steroid hormone metabolism.

DDT is still used because it’s cheap and effective at killing insects. In the absence of alternatives and where its use saves lives – as in malaria control – governments in developing countries may have little choice but to use it. DDT is a prime case for caution and each instance should be looked at carefully before application is approved. To assess any potential environmental and health impacts in Namibia I would need to know what it is used for, in what quantities and whereabouts. If you wish to pursue this further, please write to me at the address below and I shall be happy to look into it for you.

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